1. Field of the Invention
The present invention relates to chemical vapor deposition and more particularly to chemical vapor deposition of fine grained rhenium on carbon based substrates. Such are useful as light weight, high temperature strength, corrosive gas resistant structural elements for manufacturing rocket nozzles, heat exchangers and high temperature valves.
2. Description of the Prior Art
It is known in the art that a composite of carbon fibers in a carbon matrix, i.e. carbon-carbon, is a material with exceptional high temperature strength. Carbon and carbon-carbon structural elements can be used at temperatures where other refractory materials have lost their practical strength. Unfortunately, carbon is susceptible to corrosion, oxidation, and erosion when exposed to oxidizing or corrosive environments, even at moderate temperatures. Carbon is particularly vulnerable to oxidation in hostile environments such as are present in rocket exhaust gases, steam turbines and heat exchangers. The use of carbon-carbon for high temperature applications in such severe environments requires a protective coating. Rhenium is a metal that has been shown to successfully protect materials from erosive and corrosive environments. The protective quality of rhenium depends on its crystalline grain structure. Uniform, fine grained materials produce the best results. Large grain structure in metallic materials introduce stress points resulting in fracture and crack formation during temperature cycling thereby exposing the underlying carbon to corrosion. Uniform, small grains minimize the tendency for crack formation. In addition, the ductility of a protective film determines its ability to survive large temperature variations. A ductile material such as rhenium is able to conform to stresses produced by differences in rhenium and carbon substrate thermal expansions. It is known that films of rhenium can be applied by the method of chemical vapor deposition (CVD). For example, in one commercial practice, rhenium is deposited by the decomposition of gaseous ReCl.sub.5, which is formed by exposing rhenium pellets to a stream of chlorine gas at 500.degree. C. to 1200.degree. C. These high deposition temperatures result in rhenium deposits with large grain sizes, on the order of 60 micrometers. Processes that deposit rhenium at high temperature, such as from rhenium pentachloride can result in carbon contamination due to diffusion into the rhenium from the carbon substrate. Carbon impurities in rhenium are known to reduce rhenium ductility, increase film brittleness and reduce the strength of components fabricated from rhenium coated carbon-carbon composites.
It has been reported by Isobe, et al, J. Less-Common Metals, 152, 177 (1989) that a low temperature CVD process can produce fine-grained rhenium films on carbon substrates. However, this process is based on the decomposition of Re.sub.2 (CO).sub.10 which produces rhenium with a degree of carbon contamination. Rhenium precursors that contain carbon, such as rhenium carbonyl groups, should be avoided since then can directly incorporate carbon into a deposited rhenium film. The use of CVD for the hydrogen reduction of ReF.sub.6 to produce rhenium films on copper tubing has been reported by Donaldson, et al, J. Less-Common Metals, 14, 93 (1968).
It has now been unexpectedly found that the reduction of rhenium hexafluoride by hydrogen, which deposits rhenium at low or moderate temperatures is a preferred method of depositing rhenium substantially without carbon contamination. An advantage of relatively low deposition temperature is that the diffusion of carbon from the substrate into the rhenium is minimized during rhenium deposition. Rhenium on carbon structural elements produced by the method of this invention find use in applications requiring grain sizes of very small dimensions. These include diffusion bonding of adjacent rhenium sheets where small grain sizes enhance the metal diffusion rates, and braze bonding of rhenium sheets where the indiffusion of the braze metal is enhanced by rapid grain growth of deposited fine metal grains. The fine grained rhenium structure results in uniform bond formation that maintains its strength at high temperatures.